265 PRACTICAL ORGANIC CHEMISTRY and the four bonds are directed towards the four corners of the tetrahedron, this space arrangement will correspond to a doubly-linked carbon. If the two spare corners of each tetra- hedron are now occupied by different groups, it is possible to produce two forms by transposing one pair of groups. Suppos- ing A and H to represent two different groups, the above forms will result The two pairs of acids will be represented as follows :— HO-CO O-OH CO'OH O'OH Fumaric acid. Maleic acid. CO'OH O-OH Mesaconic acid. Citrac Isomerism in this case is not characterised by optical activity, as the groups lie in one plane and no structural asymmetry is possible ; but is exhibited by such physical differences as solu- bility, melting-point, electrical conductivity, and by the fact that in the case of dibasic acids only one of the pair yields an anhy- dride. Maleic and citraconic acid form anhydrides, but fumaric and mesaconic acid do not. In the case of the acids which form anhydrides, the carboxyl groups are supposed to be nearer together, i.e. on the same side (as) of the molecule, in the other case on opposite sides (trans] of the molecule. Maleic and citraconic are "cis" acids, fumaric and mesaconic are "trans"